High Temperatures Speed Corrosion in Ship Tanks DNV Study

In the turbulent world of marine transportation, corrosion in cargo holds and ballast tanks operates as an invisible threat, constantly jeopardizing vessel safety and operational efficiency. Analysis from DNV reveals that corrosion has become the most common cause of structural damage in ships. More alarmingly, surveyors frequently discover that tank coating systems fail much earlier than anticipated, with high temperatures emerging as a key accelerating factor.

The actual corrosion rates in ship ballast tanks and cargo holds often deviate significantly from industry standards and embedded corrosion models. DNV data shows that tank corrosion problems increase dramatically during oil tankers' and bulk carriers' third renewal surveys, peaking at the fourth renewal survey. This suggests some vessels may only achieve about two-thirds of the coating protection lifespan required by the Performance Standard for Protective Coatings (PSPC).

DNV materials experts emphasize that corrosion follows a nonlinear progression with more severe consequences than typically assumed. Once protective coatings begin failing, corrosion spreads rapidly, causing pitting, edge corrosion, or groove corrosion. Analysis indicates most corrosion occurs in areas suspected of high-temperature exposure, where deterioration progresses noticeably faster.

Coating system failures typically result from multiple interacting factors. Corrosion development depends on environmental conditions including salinity, acidity, humidity, temperature, exposure duration, and coating condition. These variables create highly nonlinear corrosion rates that vary significantly over time.

While paint manufacturers attribute most failures to improper surface preparation or application, marine insurers and charterers often blame incorrect specifications (design/quality flaws) or operations exceeding design limits. Examples include prolonged high-temperature cargo storage, improper cleaning procedures, and mechanical damage during cargo/ballast operations.

Among all corrosion factors, high-temperature exposure remains frequently overlooked. DNV simulation models based on actual test data show bare carbon steel's corrosion rate increases approximately 30% per 10°C temperature rise. Coated steel also suffers because heat accelerates coating aging, causing early localized failures at hard points, welds, and other vulnerable areas.

Ballast tanks adjacent to heated cargo/fuel tanks demonstrate this phenomenon. Recent DNV surveys found accelerated steel corrosion in such spaces even when previous class surveys rated their coatings as "good," proving that seemingly intact coatings struggle to prevent corrosion under thermal stress.

Corrosion creates multifaceted consequences affecting structural safety, operational costs, and environmental risks:

• Structural integrity loss: Corrosion reduces steel thickness through localized or general wastage. While pitting or groove corrosion might not immediately threaten overall safety, it promotes crack formation, holes, or surface irregularities leading to ballast tank leaks. These may cause environmental contamination or cargo pollution. Multiple components may fail through "domino effect" overloads from adjacent corroded structures.
• Increased maintenance/operational costs: Untreated corrosion raises repair expenses. Misjudging corrosion rates may necessitate unplanned steel replacement during class surveys, affecting off-hire costs or contract compliance.

Effective corrosion management requires comprehensive strategies combining coating maintenance, regular inspections, and professional assessments:

• Coating maintenance: Preserving original protective coatings remains the most effective method. Repairs must follow paint manufacturers' specifications by qualified personnel to prevent premature re-failure.
• Regular coating inspections: Systematic inspection programs enable early identification and resolution of corrosion issues.
• DNV class expertise: Early consultation allows proper tank condition evaluation. DNV offers customized hull inspection programs for tanks with reported corrosion or "fair"/"poor" coating ratings, including targeted inspections between regular surveys.

DNV's comprehensive guidelines on marine corrosion protection outline relevant methods, technical requirements, principles, and acceptance criteria. The COAT-PSPC class notation verifies proper cargo/ballast tank coating application during newbuilding stages.